Both the proton and electron of a hydrogen atom have spins. The atom has a higher energy if both are spinning in the same direction, and a lower energy if they spin in opposite directions. The amount of energy needed to reverse the spin of the electron is equivalent to a photon at the frequency of 1,420,405,751.786 Hertz (Hz) which corresponds to the 21 cm line in hydrogen spectrum.
Hydrogen masers are very complex devices and sell for as much as US$235,000. They are made in two types: active and passive.
In both types, a small storage bottle of molecular hydrogen leaks a controlled amount of gas into a discharge bulb. Molecular hydrogen consists of pairs of atoms bound together. The molecules are disassociated in the discharge bulb into individual hydrogen atoms by an arc. This atomic hydrogen passes through a collimator and a magnetic state selector. The atoms are thereby selected for the desired state and passed on to a storage bulb. The storage bulb is roughly 20 cm high and 10 cm in diameter and made of quartz. Its inside is coated with Teflon, allowing many collisions of the atoms with the wall without perturbation of the atomic state, and slows the recombination of the hydrogen atoms into hydrogen molecules. A durable Teflon & bulb coating technology allows for over 20-year lifetime. The storage bulb is in turn inside a microwave cavity made from a precisely machined copper or silver-plated ceramic cylinder. This cavity is tuned to the 1420 MHz resonance frequency of the atoms. A weak static magnetic field is applied parallel to the cavity axis by a solenoid to separate the magnetic Zeeman sublevels. To decrease the influence of changing external magnetic field on the transition line frequency and be compliant to electromagnetic interferences, the cavity is surrounded by several nested layers of shields.
In the active hydrogen maser, the cavity oscillates by itself. This requires a higher hydrogen atom density and a higher quality factor for the cavity. However, in advanced microwave cavity made out of silver-plated ceramic, the gain factor is much higher, thereby requiring less hydrogen atom density. The active maser is more complex and more expensive but has better short-term and long-term frequency stabilities. E.g., the CH1 75 model made by PTF weighs 90 kg and consumes about 100 watts of power. Its long-term frequency accuracy is about ±5×10−16 over five years. The iMaser 3000 model has enhanced short-term frequency stability of 1.2x10−13 at 1 second and a very low phase noise of -130 dBc/Hz at 1 Hz from a 5 MHz reference. Its intrinsic frequency drift demonstrates as low as 7x10−16 per day even without implementing the Automatic Cavity Tuning system. When used as a clock, it will gain or lose a second in about 63 million years.
In the passive hydrogen maser, the cavity is fed from an external 1420 MHz frequency. The external frequency is tuned to produce a maximum output in the cavity. This allows the use of lower hydrogen atom density and lower cavity quality factor, which reduces the cost. The CH1 76 model also made by PTF weighs 55 kg and consumes about 90 watts. Its long-term frequency accuracy is much worse than the active maser at about ±1.5×10−12 per year.
- "Time and Frequency from A to Z: H". Physics Laboratory. NIST. Hydrogen Maser. Retrieved 2010-04-06.
- Symmetricom MHM 2010 Data Sheet
- "iMaser 3000". Retrieved 2010-04-08.
- USNO, Hydrogen Masers
- U.S. Patent 5,838,206Active Hydrogen Maser Atomic Frequency Standard
- Bernier, Laurent-Guy; Swiss Federal Office of Metrology and Accreditation (March 2005). "Predictability of a Hydrogen Maser Time Scale" (PDF). Proceedings. 19th European Time and Frequency Forum. Besançon. pp. 438–441. Retrieved 2010-04-07.
- CHI-75 Active Hydrogen Maser Data Sheet
- CHI-76 Passive Hydrogen Maser Data Sheet